Increased blood supply triggered by the brains detection of increased toxins in the blood

this is useful for a athlete during a warm up because an increase in blood flow will supply more oxygen to the muscles 

The fluid becomes less viscous and the R.O.M. at the joint increases. This allows for a greater lubrication of the joint.

this will cause swelling in the muscles which creates pressure on the nerve endings and pain

noticeable changes will only be seen if the body has the right  fuel and rest to repair the micro tears.

when the muscles repair they will be stronger.

this is useful for a weight lifter because muscle fibre tears will result in an increased muscle mass and there for make the athlete better at there event. 

this ensures adequate delivery of oxygen and nutrients to the muscles to continue with the exercise
Oxygen demand is much higher during physical activity then it is at rest. The activity responses include an increased heart rate, increased stroke volume and increased cardiac output. All of these contribute ot an increased delivery of 02 to the working muscles during exercise.

When we take part in physical activity for a short period of time our blood pressure increased and this is because the heart is forced to pump faster and more blood is flowing around the body at a quick rate.

Increase in pressure causes increase in diffusion of gases. this allows for more 02 to be delivered to the working muscles, in addition to more C02 being removed from the blood stream which is a waste product the body does not need.